Semiconductor device

ABSTRACT

A semiconductor device according to an embodiment includes a normally-off transistor having a first drain, a first source electrically connected to a source terminal, and a first gate electrically connected to a gate terminal, a normally-on transistor having a second source electrically connected to the first drain, a second drain electrically connected to a voltage terminal, and a second gate electrically connected to the first source, a coil component provided between the voltage terminal and the second drain, and a first diode having a first anode electrically connected to the first drain and the second source, and a first cathode electrically connected to the coil component and the voltage terminal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2015-014821, filed on Jan. 28, 2015, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device.

BACKGROUND

As a material of a next generation power semiconductor device, agroup-III nitride semiconductor such as a gallium nitride-based(GaN-based) semiconductor is expected. The GaN-based semiconductor has awide bandgap, compared to silicon (Si). Therefore, a device of theGaN-based semiconductor achieves high breakdown voltage and small loss,compared to the device of Si.

In a transistor of the GaN-based semiconductor, a high electron mobilitytransistor (HEMT) structure in which two-dimensional electron gas (2DEG)is used as a carrier is generally applied. The general HEMT is anormally-on transistor that is conductive even when voltage is notapplied to a gate. There is a problem in which a normally-off transistorthat does not become conductive unless otherwise voltage is applied tothe gate is hardly achieved.

In a power circuit or the like that handles large scale power such ashundreds to one thousand volts, normally-off operation is requiredconsidering safety. Therefore, proposed is a circuit configuration inwhich the normally-off operation is achieved by cascode-connecting anormally-on GaN-based semiconductor transistor to a normally-off Sitransistor.

However, in such a circuit configuration, there may be problems in whichbreakdown of device or degradation in characteristic is caused in thecase where overvoltage occurs at a connection point between the twotransistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a semiconductor deviceaccording to a first embodiment;

FIG. 2 is a circuit diagram illustrating a switching device according toa comparative embodiment;

FIG. 3 is a circuit diagram illustrating a semiconductor deviceaccording to a second embodiment; and

FIG. 4 is a circuit diagram illustrating a semiconductor deviceaccording to a third embodiment;

DETAILED DESCRIPTION

A semiconductor device according to an embodiment includes anormally-off transistor having a first drain, a first sourceelectrically connected to a source terminal, and a first gateelectrically connected to a gate terminal, a normally-on transistorhaving a second source electrically connected to the first drain, asecond drain electrically connected to a voltage terminal, and a secondgate electrically connected to the first source, a coil componentprovided between the voltage terminal and the second drain, and a firstdiode having a first anode electrically connected to the first drain andthe second source, and a first cathode electrically connected to thecoil component and the voltage terminal.

Embodiments of the present disclosure will be described below withreference to the drawings. Note that, in the following description, asame member and the like will be denoted by a same reference sign, and adescription for a member and the like once described will be suitablyomitted.

Further, in the present specification, note that the a concept of thesemiconductor device includes: a power module formed by combining aplurality of elements such as discrete semiconductors; an intelligentpower module having the elements such as discrete semiconductorsembedded with a self-protection function and a drive circuit to drivethe mentioned elements;

or an entire system including the power module and the intelligent powermodule.

Further, in the present specification, note that an “inductor” is anelectronic component capable of storing energy in a magnetic fieldformed of flowing current. The “inductor” has the same meaning as a“coil”.

Furthermore, in the present specification, note that a “transformer”means an electronic component that converts a voltage height of AC powerby utilizing electromagnetic induction. The “transformer” includes aplurality of coils magnetically connected.

Further, in the present specification, note that a “coil component”means an electronic component including a coil. Exemplary coilcomponents are, for example, the inductor and the transformer.

In the present specification, note that “GaN-based semiconductor” is acollective term for a semiconductor including gallium nitride (GaN),aluminum nitride (AlN), indium nitride (InN), and an intermediatecomposition thereof.

First Embodiment

A semiconductor device according to an embodiment includes: anormally-off transistor having a first drain, a first sourceelectrically connected to a source terminal, and a first gateelectrically connected to a gate terminal, a normally-on transistorhaving a second source electrically connected to the first drain, asecond drain electrically connected to a voltage terminal, and a secondgate electrically connected to the first source, a coil componentprovided between the voltage terminal and the second drain, and a firstdiode having a first anode electrically connected to the first drain andthe second source, and a first cathode electrically connected to thecoil component and the voltage terminal.

The semiconductor device according to the present embodiment has thecoil component that is an inductor.

Further, the semiconductor device according to the present embodimentfurther includes a second diode having a second cathode and a secondanode electrically connected between the second drain and the inductor,and a capacitor electrically connected to the second cathode.

The semiconductor device according to the present embodiment is a powerconverter. The semiconductor device according to the present embodimentis, specifically, a non-isolated boosting chopper circuit that boostspower voltage Vdd. In the non-isolated boosting chopper circuitaccording to the present embodiment, a circuit connecting a normally-ontransistor and a normally-off transistor in series is used as aswitching device.

FIG. 1 is a circuit diagram illustrating a semiconductor deviceaccording to the present embodiment.

In the switching device of the semiconductor device according to thepresent embodiment, a normally-off transistor 10 and a normally-ontransistor 20 are connected in series. The normally-off transistor 10and the normally-on transistor 20 implement a normally-off switchingdevice by a so-called cascode connection.

The normally-off transistor 10 is, for example, a vertical metal oxidesemiconductor field effect transistor (MOSFET) of silicon (Si). Further,the normally-on transistor 20 is, for example, an HEMT of a galliumnitride-based (GaN-based) semiconductor. The normally-on transistor 20includes a gate insulating film.

Note that the normally-off transistor 10 includes a parasitic body diodenot illustrated.

The normally-off transistor 10 has low element breakdown voltage,compared to the normally-on transistor 20. The normally-off transistor10 has the breakdown voltage of 10 V or more and 30 V or less, forexample. Further, the normally-on transistor 20 has the breakdownvoltage of 600 V or more and 1200 V or less, for example.

The semiconductor device includes a source terminal 100, a voltageterminal 200, and a gate terminal 300. The normally-off transistor 10includes a first source 11 connected to the source terminal 100, a firstdrain 12, and a first gate 13 connected to the gate terminal 300. Thenormally-on transistor 20 includes a second source 21 connected to thefirst drain 12, a second drain 22 connected to the voltage terminal 200,and a second gate 23 connected to the first source 11.

The semiconductor device includes an inductor 35 as the coil component.The inductor 35 is provided between the voltage terminal 200 and thesecond drain 22.

The semiconductor device includes a first diode 50 having a first anode51 and a first cathode 52. The first anode 51 is connected to the firstdrain 12 and the second source 21. The first cathode 52 is connected tothe inductor 35 and the voltage terminal 200.

The semiconductor device includes a second diode 60 having a secondanode 61 and a second cathode 62. The second anode 61 is connected tothe second drain 22 and the inductor 35.

Further, the semiconductor device includes a capacitor 55. The capacitor55 has one end connected to the second cathode 62. The capacitor 55 hasthe other end fixed to, for example, ground potential.

For example, an output terminal 400 is provided on the second cathode 62side of the second diode 60.

In the following, operation of the semiconductor device according to thepresent embodiment will be described.

When the switching device according to the present embodiment formed ofthe normally-off transistor 10 and the normally-on transistor 20 is inan ON state, 0 V is applied to the source terminal 100 and positivevoltage is applied to the second drain 22. Further, positive voltagesuch as 10 V is applied to the gate terminal 300.

At this point, positive voltage is applied to the first gate 13 of thenormally-off transistor 10. Therefore, the normally-off transistor 10performs ON operation.

On the other hand, the second gate 23 of the normally-on transistor 20is clamped at the source terminal 100 via the first diode 50. Therefore,voltage at the second gate 23 becomes 0 V.

The second source 21 comes to have potential close to 0 V by turning onthe normally-off transistor 10. Therefore, the normally-on transistor 20also comes to perform the ON operation. Accordingly, ON current flowsbetween the source terminal 100 and the second drain 22.

Next, a case where the switching device is changed from an ON state toan OFF state will be studied. In this case, applied voltage to the gateterminal 300 is changed from positive voltage to 0 V or negativevoltage. The gate terminal 300 is decreased to 0 V from 10 V, forexample.

First, 0 V is applied to the first gate 13 of the normally-offtransistor 10. Therefore, the normally-off transistor 10 performs OFFoperation.

In the case where the switching device shifts from the ON state to theOFF state, the normally-off transistor 10 is first turned off and thenvoltage at a connecting portion between the normally-off transistor 10and the normally-on transistor 20 is increased. Next, when a potentialdifference between the second source 21 and the second gate 23 clampedat 0 V reaches threshold voltage, the normally-on transistor 20 performsOFF operation. The connecting portion between the normally-offtransistor 10 and the normally-on transistor 20 corresponds to the firstdrain 12 of the normally-off transistor 10 and the second source 21 ofthe normally-on transistor 20.

The normally-on transistor 20 performs OFF operation, following thenormally-off transistor 10. Therefore, current between the sourceterminal 100 and the second drain 22 is shut off.

The switching device according to the present embodiment functions asthe normally-off transistor in which the source terminal 100 is asource, the second drain 22 is a drain, and the gate terminal 300 is agate.

When the switching device is in the ON state, current flows from thevoltage terminal 200 to the source terminal 100. The current also flowsin the inductor 35. Even when the switching device shifts from the ONstate to the OFF state, the current continues flowing in the inductor 35for a predetermined period. Therefore, even when the switching device isin the OFF state, the current passes through the second diode 60 andflows to the output terminal 400 side.

The power voltage Vdd is applied to the voltage terminal 200. Theswitching device is made to perform ON/OFF operation at a predeterminedduty ratio, thereby transmitting energy stored in the inductor 35 to theoutput terminal 400 side. Then, the power voltage Vdd turns to boostedoutput voltage.

The power voltage Vdd is, for example, 10 V or more and 50 V or less.Further, the boosted output voltage is, for example, 50 V or more and200 V or less.

According to the semiconductor device according to the presentembodiment, in the case where the voltage at the connecting portionbetween the normally-off transistor 10 and the normally-on transistor 20becomes higher than the power voltage Vdd, current flows to the voltageterminal 200 from the first drain 12 and the second source 21 byproviding the first diode 50.

Next, functions and effects of the semiconductor device according to thepresent embodiment will be described.

FIG. 2 is a circuit diagram illustrating a switching device according toa comparative embodiment. The semiconductor device according to thecomparative embodiment has a circuit configuration in which anormally-off transistor 10 is cascode-connected to a normally-ontransistor 20. The normally-off transistor 10 and the normally-ontransistor 20 are transistors same as the present embodiment.

This switching device includes a source terminal 100, a voltage terminal200, and a gate terminal 300. Further, the normally-off transistor 10includes a first source 11 connected to the source terminal 100, a firstdrain 12, and a first gate 13 connected to the gate terminal 300.Furthermore, the normally-on transistor 20 includes a second source 21connected to the first drain 12, a second drain 22 connected to thevoltage terminal 200, and a second gate 23 connected to the first source11.

The switching device according to the comparative embodiment alsofunctions as a normally-off transistor in which the source terminal 100is a source, the voltage terminal 200 is a drain, and the gate terminal300 is a gate.

In the circuit configuration of the comparative embodiment, there may bepossibility that overvoltage may occur at a connecting portion betweenthe normally-off transistor 10 and the normally-on transistor 20 duringdevice operation. The connecting portion between the normally-offtransistor 10 and the normally-on transistor 20 corresponds to the firstdrain 12 of the normally-off transistor 10 and the second source 21 ofthe normally-on transistor 20.

For example, when the switching device shifts from an ON state to an OFFstate, transient current is generated, and high voltage applied betweenthe source terminal 100 and the voltage terminal 200 is divided at aratio of parasitic capacitance between the normally-off transistor 10and the normally-on transistor 20, thereby causing overvoltage.

In the case of the comparative embodiment, when the state shifts fromthe ON state to the OFF state, the normally-off transistor 10 is turnedoff and then voltage is first boosted at the connecting portion.Subsequently, the normally-on transistor 20 performs OFF operation whena potential difference between the second source 21 and the second gate23 clamped at 0 V reaches threshold voltage. Therefore, when thepotential at the connecting portion rises due to the transient current,overvoltage occurs at the connecting portion because there is no passageto release charge.

When overvoltage occurs, high voltage is applied between the secondsource 21 and the second gate 23 of the normally-on transistor 20. Whenthis overvoltage reaches breakdown voltage or more of a gate insulatingfilm, leak current at the gate insulating film of the normally-ontransistor 20 is increased. Or, there may be possibility that the gateinsulating film is broken. When leak current is increased at the gateinsulating film of the normally-on transistor 20 or when the gateinsulating film is broken, the semiconductor device including theswitching device malfunctions. Therefore, reliability of thesemiconductor device is degraded.

Further, even in the case where no problem occurred at the gateinsulating film, charge is trapped to the second source 21 side by thehigh voltage applied between the second source 21 and the second gate 23of the normally-on transistor 20. Such trapping of charge may causecurrent collapse. When current collapse occurs, ON current is decreased,thereby causing malfunction. Therefore, reliability of the semiconductordevice is also degraded.

The semiconductor device according to the present embodiment includesthe first diode 50 having the first anode 51 and the first cathode 52.The first anode 51 is connected to the first drain 12 and the secondsource 21. The first cathode 52 is connected to the inductor 35 and thevoltage terminal 200. In other words, the connecting portion of theswitching device is connected to the voltage terminal 200 interposingthe first diode 50.

Therefore, even when overvoltage is applied to the first drain 12 andthe second source 21, current flows to the voltage terminal 200 from thefirst drain 12 and the second source 21 when overvoltage exceeds thepower voltage Vdd. Therefore, overvoltage at the connecting portion issuppressed. As a result, reliability of the semiconductor device isimproved.

Further, according to the present embodiment, when overvoltage occurs atthe connecting portion of the switching device, current flows to thevoltage terminal 200 from the connecting portion. By this, excessivecharge induced at the connecting portion is returned to the powersource. Therefore, energy is regenerated and energy efficiency in theboosting chopper circuit is improved.

As described above, according to the present embodiment, thesemiconductor device having improved reliability can be implemented.Further, the semiconductor device having high energy efficiency isimplemented.

Second Embodiment

A semiconductor device according to the present embodiment is same as afirst embodiment except for that a resistance element is furtherprovided between a gate terminal and a first gate. Therefore, repetitionof the same description as the first embodiment will be omitted.

FIG. 3 is a circuit diagram illustrating the semiconductor deviceaccording to the present embodiment.

The semiconductor device according to the present embodiment includes aresistance element 75 provided between a gate terminal 300 and a firstgate 13.

In a power electronics circuit design, there may be a case whereadjustment of an operation speed of a transistor is demanded for thepurpose of noise prevention. According to the present embodiment,transmission of gate voltage applied to the gate terminal 300 to thefirst gate 13 and a second gate 23 can be delayed by providing theresistance element 75. Therefore, an operation speed (switching speed)of the semiconductor device can be adjusted.

According to the present embodiment, adjusting the operation speed ofthe semiconductor device (switching speed) can be performed in additionto effects of the first embodiment.

Third Embodiment

A semiconductor device according to the present embodiment has a sameconfiguration as a first embodiment except for that a coil component isa transformer and a second diode 60 and a capacitor 55 constituting aboosting chopper circuit are not included. Therefore, repetition of thesame description as the first embodiment will be omitted.

The semiconductor device according to the present embodiment is a powerconverter. The semiconductor device according to the present embodimentis, specifically, a transformer (voltage transformer) that transformsvoltage of power voltage Vdd. In the transformer according to presentembodiment, a circuit in which a normally-on transistor and anormally-off transistor are connected in series is used as a switchingdevice.

FIG. 4 is a circuit diagram illustrating the semiconductor deviceaccording to the present embodiment.

According to the semiconductor device according to the presentembodiment, a transformer 80 is provided between a voltage terminal 200and a second drain 22 of a normally-on transistor 20. The transformer 80includes a primary coil 81 and a secondary coil 82. Further, asillustrated in FIG. 4, the secondary coil 82 of the transformer 80 isconnected to, for example, a diode 85, a capacitor 90, and a loadresistance 95.

The semiconductor device according to the present embodiment transformsthe power voltage Vdd applied to the voltage terminal 200 by using thetransformer 80. The power voltage Vdd is, for example, 10 V or more and50 V or less. Further, the transformed voltage is 50 V or more and 200 Vor less, for example.

The semiconductor device according to the present embodiment includes afirst diode 50 having a first anode 51 and a first cathode 52 same asthe first embodiment. Further, the first anode 51 is connected to afirst drain 12 and a second source 21. The first cathode 52 is connectedto the transformer 80 and the voltage terminal 200. In other words, thefirst drain 12 and the second source 21 are connected to the voltageterminal 200, interposing the first diode 50.

Therefore, even when overvoltage is applied to the first drain 12 andthe second source 21, current flows to the voltage terminal 200 from thefirst drain 12 and the second source 21 when overvoltage exceeds thepower voltage Vdd. Therefore, overvoltage at a connecting portion issuppressed. As a result, reliability of the semiconductor device isimproved.

Further, according to the present embodiment, when overvoltage occurs atthe connecting portion of the switching device, current flows to thevoltage terminal 200 from the connecting portion. By this, excessivecharge induced at the connecting portion is returned to the powersource. Therefore, energy is regenerated, and energy efficiency of theboosting chopper circuit is improved.

As described above, according to the present embodiment, thesemiconductor device having improved reliability can be implemented sameas the first embodiment. Further, the semiconductor device having highenergy efficiency is implemented.

According to the first to third embodiments, the case where thesemiconductor device is the boosting chopper circuit and the transformer(voltage transformer) has been described as an example, but the presentdisclosure can be also applied to a relay circuit in which a coilcomponent is an inductor, for example.

Further, the case where the normally-on transistor 20 is the HEMT of thegallium nitride-based (GaN-based) semiconductor has been described as anexample. However, a transistor using other wide bandgap semiconductorssuch as silicon carbide (SiC) and diamond can also be applied to thenormally-on transistor 20.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the devices andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A semiconductor device, comprising: anormally-off transistor having a first drain, a first sourceelectrically connected to a source terminal, and a first gateelectrically connected to a gate terminal; a normally-on transistorhaving a second source electrically connected to the first drain, asecond drain electrically connected to a voltage terminal, and a secondgate electrically connected to the first source; a coil componentprovided between the voltage terminal and the second drain; and a firstdiode having a first anode electrically connected to the first drain andthe second source, and a first cathode electrically connected to thecoil component and the voltage terminal.
 2. The device according toclaim 1, wherein the normally-on transistor is an HEMT of a GaN-basedsemiconductor.
 3. The device according to claim 1, wherein the coilcomponent is an inductor.
 4. The device according to claim 1, whereinthe coil component is a transformer.
 5. The device according to claim 3,further comprising: a second diode having a second anode electricallyconnected to the second drain and the inductor, and a second cathode;and a capacitor electrically connected to the second cathode.
 6. Thedevice according to claim 1, further comprising a resistance elementprovided between the gate terminal and the first gate.
 7. The deviceaccording to claim 1, wherein the normally-off transistor is a MOSFET ofSi.
 8. The device according to claim 2, wherein the coil component is aninductor.
 9. The device according to claim 5, wherein the normally-ontransistor is an HEMT of a GaN-based semiconductor.
 10. The deviceaccording to claim 9, wherein the normally-off transistor is a MOSFET ofSi.